1. Field of the Invention
The present invention relates to an information recording medium and, more particularly, to an optical disk memory such as a compact disk (CD), a magnetooptical disk (MO), and the like, from which information can be read out using light.
2. Related Background Art
Optical disks are classified into read-only type disks on which information is recorded in advance, and recording/reproducing type disks on which a user can desirably record signals. The recording/reproducing type disks are further classified into a write-once type disk on which permanent recording is performed, and an erasable type disk on which a new signal can be recorded by erasing recorded information. Typical read-only type optical disks include audio disks (CD) and video disks for home use.
On a recording/reproducing type optical disk, recording is attained by irradiating a laser beam modulated by a signal onto a rotating disk to locally change the state of a recording thin film. The recorded signal is reproduced by irradiating a weak constant light beam which does not change the thin film and detecting the change in light reflected by the disk. Such an optical disk is practically applied to a video image file and document file for business use, and an external memory of a personal computer.
FIG. 6 shows the structure of a normal CD. When this CD is inserted into a CD player, audio data is reproduced. In this CD, relief-like phase pit arrays 82 are spirally formed on a 1.2-mm thick transparent substrate 81. The respective phase pit arrays serve as information tracks. More specifically, audio information is recorded in the CD as the lengths of phase pits.
CDs are popularly used as information recording media in audio equipments and computers. Such an information recording medium is described in U.S. Pat. No. 4,423,502.